Electro-optic modulators find significant use in fiber optic communication systems to modulate data onto a light beam. Light is conducted through a waveguide and electrical signals are applied across the waveguide to apply a modulation to the light beam. Lithium niobate is a common material for such modulators because it works well as a waveguide for light and also exhibits excellent properties for modulating light with electrical inputs. However, lithium niobate is not a good material for semiconductor circuitry. As a result, a lithium niobate modulator requires a separate chip, typically silicon, to carry circuitry for the phase modulator. Electrical wires connect the two silicon chip outputs to the lithium niobate chip. In addition, modulating light with a lithium niobate modulator requires typically a swing of 5 V; a swing that modern CMOS processes cannot easily supply. A much less expensive modulator may be made by putting both the circuitry and the waveguide on the same chip, however, silicon is not a good material for a light waveguide.
In order to have a long interaction length between the electrical signal and optical wave in an electro-optic modulator, most electro-optic modulators include a long waveguide. An example of such a waveguide is a Mach-Zehnder lithium niobate modulator. Such a modulator will normally have electro-optic material (i.e. lithium niobate) embedded in an electrical waveguide with the modulating electrical field penetrating the electro-optic material of the optical waveguide. The electrical waveguide will typically have a characteristic impedance of about 50 Ohms and can be driven by a 50 Ohm driver.
Semiconductor modulators constructed similar to a Mach Zehner lithium niobate modulator have a much higher capacitance per unit length. This creates a much higher electrical load and a much slower phase velocity. The low phase velocity can cause a phase mismatch between the optical and the electrical wave as the two waves travel along the waveguide. One effect of the phase mismatch is to limit the maximum data rate of the communication system that uses the semiconductor modulator.
It is very difficult to change the inductance per unit length of a semiconductor waveguide by changing its geometry, that is the shape, depth, width, or length of the waveguide. The inductance is only weakly dependent on the geometry of the waveguide and normally will remain within a range of 1 to 1.3 nH/mm. Changing the waveguide geometry also has other significant effects on the modulator and the geometry cannot be changed freely without considering the many other factors that are affected.